Swarming is a new property rune for thrown weapons from Treasure Vault. It can be etched on a Thrown weapon. As a 2 action activity once per hour, you can do 41 damage in 30-foot cone.
I expect more from a 9th level rune, that is only usable once per hour. On this level, this is the minimum damage of martial classes (2dx+0+2, Striking rune + Str bonus + Weapon Specialization), and if they are serious about throwing, they can go as high as 13 (2d8+4+2 average). Admittedly, this damages multiple targets, but takes at least 3 just to break even.
Unlike elemental runes, the usefulness of this quickly falls behind, as the DC is fixed. By the time it gets 6 damage (Greater Striking, level 12) the expected Ref save of monsters increases by 4. When it gets to 8 damage, most monsters will critically save most of the time.
Is it really that bad, or am I missing something?
- Twice the damage dice, assuming you bought a Striking rune on level 4, as you should